Fix multi-process race in Wrapper.Start (LOCALDB_ERROR_INSTANCE_DOES_NOT_EXIST)#929
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SimonCropp merged 6 commits intoSimonCropp:mainfrom May 6, 2026
Merged
Fix multi-process race in Wrapper.Start (LOCALDB_ERROR_INSTANCE_DOES_NOT_EXIST)#929SimonCropp merged 6 commits intoSimonCropp:mainfrom
SimonCropp merged 6 commits intoSimonCropp:mainfrom
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Implement serialization of the `Wrapper.Start` method for LocalDB instances using both in-process and cross-process locks. For in-process synchronization, employ a `ConcurrentDictionary` to manage `SemaphoreSlim` instances keyed by instance name, ensuring that two `Wrapper` instances within the same process don't race on the same LocalDB instance. For cross-process synchronization, use a lock file in `%TEMP%` with `FileShare.None`, preventing other processes from starting until the current instance has completed its startup task. This fix addresses the concurrency issues demonstrated by `ConcurrentStartTests` in single and multi-process scenarios. Such synchronization ensures no template rebuilds occur unexpectedly, avoiding potential races during internal state setup. The `WrapperTests` suite remains passing, confirming that existing functionality is preserved. These changes are motivated by the need to prevent the awkward interactions that can arise when multiple processes or threads attempt to interact with the same LocalDB instance simultaneously, posing risks of database corruption or test flakiness.
This reverts commit f15cf15.
This was referenced May 6, 2026
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Summary
Wrapper.Startis unsafe under concurrent calls — both within one process and across processes — whentemplate.mdfis missing. Symptoms include:0x89C50107(LOCALDB_ERROR_INSTANCE_DOES_NOT_EXIST)CREATE DATABASE [template]template.mdf"file not found" errorsReal-world trigger: two
dotnet testhosts (e.g. CLI + Rider) sharing a LocalDB user instance. Once the wrapper directory is empty, everyStarthits the destructiveStopAndDelete+CleanStartbranch and races every other process doing the same.Fix
Serialize
Wrapper.Startper instance name with two layers, both held untilstartupTaskcompletes:ConcurrentDictionary<string, SemaphoreSlim>keyed by instance name. (The old per-instancesemaphoreSlimfield was declared but never used.)FileStreamopened withFileShare.Noneon%TEMP%\localdb_wrapper_<instanceName>.lock. Used in preference toMutexbecause file handles aren't thread-affine, so the lock survivesawait.Tests added
ConcurrentStartTests— single-process race (deterministic SQL deadlock without the fix).MultiProcessConcurrentStartTests— symmetric multi-process race using a newLocalDb.MultiProcessHelperexe spawned viaProcess.Start.InstanceDoesNotExistRaceTests— asymmetric killer/victim child processes that deterministically reproduce the exact0x89C50107error code.All three race tests fail without the fix and pass with it. The 23 existing
WrapperTestsstill pass.See
src/LocalDb.MultiProcessHelper/README.mdfor the full root-cause writeup and per-piece rationale.